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Freezing Point Depression / Boiling Point Elevation Calculator

Find how much a dissolved solute shifts a solvent's freezing or boiling point, using the colligative-property formula ΔT = i × K × m.

Freezing point depression (ΔT)
1.86 °C
Solution's resulting temperature
-1.86 °C

How it works

Dissolving a solute shifts a solvent's freezing and boiling points by an amount that depends only on how many solute particles are present per kg of solvent, not on what the solute actually is — a colligative property. Both freezing point depression and boiling point elevation follow the same relationship, ΔT = i × K × m: m is the solute's molality (mol/kg), i is the van 't Hoff factor (how many particles one formula unit splits into in solution — 1 for sucrose, 2 for NaCl, 3 for CaCl₂), and K is the solvent's own cryoscopic constant (Kf, for freezing) or ebullioscopic constant (Kb, for boiling).

K is a property of the solvent, not something this calculator invents or hardcodes — water's commonly cited values (Kf = 1.86 °C·kg/mol, Kb = 0.512 °C·kg/mol) are supplied here only as the default to check the math against, and any other solvent's own K can be entered instead. Freezing point depression subtracts ΔT from the pure solvent's freezing point; boiling point elevation adds it to the pure solvent's boiling point.

FAQ

Why does road salt melt ice?

Dissolving salt into the water on a road lowers its freezing point below 0°C (exactly this ΔT = i·K·m effect, with i ≈ 2 for NaCl), so water that would otherwise refreeze at 0°C stays liquid down to a lower temperature — it doesn't make the ice itself warmer, it just shifts the temperature at which that salty water can freeze.

Why does NaCl use i = 2 but sucrose uses i = 1?

The van 't Hoff factor counts how many particles one dissolved formula unit actually becomes in solution. NaCl dissociates into a Na⁺ ion and a Cl⁻ ion, so one formula unit becomes two particles (i = 2), each contributing to the colligative effect. Sucrose is a molecular (non-electrolyte) solute that stays intact in solution, so one dissolved molecule stays one particle (i = 1).

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